This invention relates to a new soft-rubbing adhesive stick for applying an adhesive film having permanently tacky, pressure-sensitive adhesive properties.
The literature has long contained numerous proposals for the production of adhesives in stick form which are said to be suitable for bonding a variety of substrates, but especially paper. Practical significance has been acquired above all by a certain type of adhesive sticks such as those which contain an aqueous polyvinyl pyrrolidone solution as adhesive component accommodated in a soap gel, particularly sodium stearate, as the shaping gel-forming component. Sticks of this type are described in particular in German Patent No. 18 17 985. U.S. Pat. No. 3,576,776 and its reissue No. Re. 30747 correspond to this German patent.
There has been no shortage of attempts to replace the soap gel forming the rigid stick substance by other gel-forming components. German Patent No. 22 04 482 proposes using the reaction product of sorbitol and benzaldehyde as the gel-forming component, particular importance being attributed to dibenzal-sorbitol. So far as the adhesive resins are concerned, the disclosure of this patent is in the same category as the teaching of the above-mentioned German Patent No. 18 17 985. In this case, too the adhesive resins used are tacky solutions, thickened to stick consistency, obtained by dissolving synthetic resins in water or in aqueous mixtures of organic solvents.
The adhesive components used in the above-mentioned patents receive their tackiness through the mixing or dissolution of the adhesive resin with or in the solvent or solvent mixture. If the solvent evaporates or escapes otherwise from the adhesive mass, for example through absorption of the liquid components into the coated paper, the adhesive-containing mass loses its tackiness. Accordingly, only a limited amount of time is available for applying adhesive coatings of the type applied by rubbing, for example onto paper, union with the material to be bonded having to take place within that time.
Adhesive resin components having fundamentally different properties, namely permanently tacky pressure-sensitive adhesive properties, are known from the technology of adhesives. Although adhesive mixtures based on permanently tacky resins such as these also generally contain more or less limited quantities of, for example, an aqueous solvent phase for the application process, no real significance is attributed to this solvent content. The adhesive or adhesive mixture retains its pressure-sensitive adhesive properties even when the liquid phase has escaped from the adhesive layer applied. Adhesive systems of this type are based on permanently tacky resin components which are distinguished by extreme softness and high tack, even in the solvent-free state, and which nevertheless are capable of forming sufficiently firm bonds. By suitably formulating the adhesive layer, provision can be made on the one hand for the formation of temporary bonds and, on the other hand, for the formation of such firm bonds that paper bonds, for example, can only be destroyed by tearing the paper.
Pressure-sensitive adhesive resins of the type in question with their permanently tacky adhesive properties have never been proposed before for the production of adhesive sticks. This is not surprising. A crucial requirement for the conversion and use of adhesives in stick form is a comparatively high hardness value of the adhesive composition which ensures that the stick as such does not change its shape in use, the adhesive being rubbed off from the stick at its end. However, the serviceability of a pressure-sensitive contact adhesive and particularly its tack are critically dependent on the soft tackiness of adhesive resin components of the type in question here.